Nitrogen oxide emissions NOx are also a consequence of the operating parameters of the combustion process but are due in principle to the use of air, which comprises virtually 80% of nitrogen N, as oxidant. The nitrogen oxide emissions in particular are, because of their potential for the formation of acid rain and summer smog, subject matter of legal limits which are becoming more demanding in steps and are partially achieved in the case of new plants by means of optimized combustion processes but in the case of existing plants require after-treatment of the flue gases.
For the present purpose, the expression NOx refers to the nitrogen monoxide NO which is initially formed in the combustion together with the nitrogen dioxide NO2 to which the NO is oxidized by slow reactions in air.
Increased nitrogen oxide emissions NOx are found particularly in the case of high combustion temperatures and long gas residence times at high temperatures, while high carbon dioxide emissions CO2 occur, inter alia, in the case of very lean combustion with an insufficient gas residence time under conditions for complete oxidation of the fuel, and are then often associated with an efficiency decrease compared to optimal combustion.
Existing power stations are, in order to reduce the nitrogen oxide emissions NOx, equipped with, inter alia, facilities for carrying out the selective catalytic reduction (SCR) of the nitrogen oxides NOx with ammonia NH3 as reducing agent; these facilities are designed for over 90% of the nitrogen oxides NOx occurring as nitrogen monoxide NO due to process conditions. Catalysts used are typically TiO2N2OF1O3 mixtures which have various mixing ratios and selectively absorb ammonia NH3 and reduce nitrogen monoxide NO in a catalytic surface reduction according to the overall reaction equation:4 NO+4 NH3+O2→4 N2+6 H2O  (a)
This reaction proceeds successfully in a temperature range from 250° C. to 450° C. Below 250° C., the reaction very quickly becomes slower because of the energy required for activation. Above 450° C., the catalytic oxidation of NH3 firstly to N2O and finally to NO commences.
For this reason, in order to effect a decrease in oxides of nitrogen, it has been proposed in EP 0 753 701 B1 that, for use in heating boilers, for example, a device be arranged between a high-temperature preheater and a low-temperature preheater in a steam generator in order to achieve the appropriate temperature range for decreasing the nitrogen oxide. However, such an arrangement increases the construction volume of a heating boiler considerably, because firstly surfaces for the heat exchanger and secondly surfaces or volumes for the NOx reduction have to be provided. Here, it is immaterial whether the reduction is carried out catalytically or not catalytically. In the case of a catalytic reduction, large surface areas on which a rapid reduction according to equation (a) occurs have to be provided. In the case of a noncatalytic reduction, a series of slow volume reductions, which compared to the catalytic reduction also have the disadvantage of a significantly lower selectivity and a high risk of formation of NO2 as by-product, occur in a comparatively narrow temperature interval at a comparatively significantly higher temperature level. NO2 is a greenhouse gas which can be degraded only slowly and has approximately 40 times the greenhouse potential of CO2.
It was proposed in EP 1 820 560 A1 that the surfaces made available by heat exchangers of a heating boiler or a waste heat steam generator of a gas turbine be made usable for flue gas purification, in particular for the selective catalytic reduction of nitrogen oxides and the oxidation of carbon monoxide CO by coating with catalytic material. However, a closer study of this proposal indicates that in the case of a waste heat steam generator the surfaces made available at temperature levels in the range from 250° C. to 450° C. are not sufficient for lasting, reliable reduction of the nitrogen oxides of significantly over 50% even when coated with highly active nanoparticulate catalysts. Robust catalysts having a long life have, compared to the nanoparticulate catalysts, a significantly smaller internal surface area and thus a lower activity and therefore allow degrees of reduction of only 30% or less from the beginning.
The low degrees of reduction of robust catalysts having a long life have various causes. In the case of powder catalysts, the active surface area is from about 45 to 60 m2/g. In the case of catalytic coating of metallic surfaces of heat exchangers, on the other hand, significantly smaller values of the active surface area have to be expected. In addition, the catalytic reactions are in this case limited by transport processes, so that only a fraction of the active catalytic surface areas is actually utilized. Furthermore, in the case of waste heat steam generators a considerable part of the surfaces is at a temperature level below 250° C.
It is known from WO 99/39809 and EP 1 147 801 A1 that, in the field of exhaust gas purification for internal combustion engines, in particular diesel engines, more efficient reduction of nitrogen oxides NO can be achieved by firstly passing the exhaust gas over an oxidation catalyst which typically oxidizes from 30 to 70% of the NO to NO2, then adding NH3 as reducing agent and passing the exhaust gas admixed with reducing agent over an SCR catalyst. This effect, also known as “fast SCR reaction”, proceeds at significantly lower temperatures than the SCR reaction. However, for thermodynamic reasons, the oxidation catalyst oxidizes NO efficiently to NO2 only below a temperature of 400° C.
FIG. 1 shows a thermodynamic equilibrium calculation for NO and NO2. The concentration in percent by volume is plotted against the temperature in kelvin. According to this, the maximum achievable degree of conversion decreases with increasing temperature. At 400° C., the degree of conversion is below 50%; at 450° C. at below 40%. The thermodynamic limit value can be achieved only when using very large reactors and is therefore not realistic in practice. Owing to limited reaction rates, a conversion of typically only 70% of the thermodynamic limit value can be expected in a compact reactor.
In combined gas and steam power stations (GaS), the temperature of the gas entering the waste heat steam generators is in the range from 450° C. to 500° C. The benefit gained from the fast SCR reaction for reducing the NOx emissions would therefore be relatively small since the NO2 would have been consumed completely at relatively high temperatures and the relatively low temperatures at which it would bring the greatest benefit would no longer be available.